Many potentially useful drugs are too toxic to be administered to a patient at the desired dose. One method for achieving the desired advantages while reducing the overall toxicity of the drug to the patient is referred to as photodynamic therapy (PDT). In PDT, a second drug that will enter the site of interest is administered to the patient. The second drug has three properties. First, the second is less toxic than the drug of interest, and hence, is tolerated better by the patient. The second drug can be converted to the first drug at the site of interest by exposing the second drug to light. Third, the amount of the first drug that escapes the site of interest after conversion of the first drug is less than the toxic limit to which the patient can be exposed once the drug is diluted by diffusing through the patient's body.
Most of the treatment sites of interest are internal to the human body. In addition, the wavelengths of light needed to provide the conversion of the first drug are outside the range of wavelengths that can be delivered through the skin and intervening tissue. Hence, PDT usually requires a light source that can be inserted into the body and placed adjacent to the site of interest.
Solid state light sources are particularly attractive candidates for PDT light sources. Solid state light sources such as LEDs and lasers emit light in narrow wavelength ranges. The particular emission wavelengths are determined by the materials and structure of the solid state devices. The conversion of the first drug to the second drug often displays an optimum range of wavelengths. It should be noted that light that is not consumed in the conversion of the first drug is eventually deposited in the patient's body in the site of interest in the form of heat. Hence, a high conversion efficiency is important to assure that the patient will not be subjected to excess heat.
One class of prior art light source for use in PDT is referred to as a “light bar”. The light bar consists of a substrate having a plurality of LEDs attached thereto. The LEDs are encapsulated in a transparent medium. Two electrical leads extend from the light bar and are used to provide power to the device. The light bar is inserted into the patient through a surgically created incision. The leads may or may not be partially within the patient's body.
This arrangement has a number of problems. First, the patient can be subjected to excessive heat. While LEDs have a high conversion efficiency in terms of the conversion of electrical power to light of the desired wavelength relative to conventional light sources such as incandescent bulbs and fluorescent lights, most of the electrical power is still converted to heat. Hence, the light bar can become hot enough to cause discomfort to the patient. To prevent such discomfort, the treatment times must be extended to accommodate the lower intensities of heat, or the concentration of the first drug must be increased. The second option is not always feasible, as the first drug may also have a toxicity problem.
Second, there are lower limits on the size of the light bar. The light bar must be large enough to accommodate the LEDs, substrate, and encapsulating material. The number of LEDs can be relatively large, which leads to a device that requires a correspondingly large incision and the medical problems associated with that incision.
Third, there is a limit to the intensity of light that can be provided by such an arrangement. There is a limit to the density of LEDs mounted on a substrate that is set by the size of the LEDs and the amount of heat that must be removed. Since each LED also has a limit on the amount of light generated, the light bar has a maximum light output per unit area. This limit can place restrictions on the types of drugs that can be activated.
Fourth, the light bar must be sterilized. The encapsulation process can provide a reasonable level of sterilization when the device is first made. However, after the bar has been in a patient, it must be re-sterilized, preferably in an autoclave that subjects the light bar to high temperature steam. This process can lead to delaminating the encapsulation layer or stresses on the internal leads. As a result, light bars are typically discarded after use. As a result, the entire device, including the LEDs and wiring must be replaced, which increases the cost of the procedure.